This invention relates to lasers and more particularly to Raman lasers or Raman cells driven by laser sources. The invention further relates to Nd:YAG driven Raman lasers.
Advanced ranging or tracking devices use reflected light from a laser source in order to ascertain target and range data for many applications. It is obviously desirable to operate such devices at wavelengths which are considered "eye-safe" in order to reduce risk to personnel encountering the direct or reflected beams of laser light.
The term "eye-safe" is applied to radiation that does not, in general, cause tissue damage to the human eye. It is known that for wavelengths between 400 and 1400 nanometers, radiation tends to cause retinal damage; while for wavelengths longer than 1400 nanometers, the radiation is absorbed within or near the surface of the cornea and much higher levels of radiation can be tolerated before reaching the level that can cause corneal damage.
Research in the area of laser safety has led to the development of specific standards for "eye-safe" lasers. The "Regulations for the Administration and Enforcement of the Radiation Control for Health and Safety Act of 1968", published by the U.S. Department of Health, generally supports the wavelength of 1.54 .mu.m as the standard for "eye-safe." The standards for lasers operating at this wavelength allow several orders of magnitude greater output energy and power than for non "eye-safe" lasers.
Raman lasers utilizing a Raman scattering medium can be used to convert laser radiation of one wavelength to a longer wavelength. This allows lasers operating at non "eye-safe" wavelengths to produce radiation within the "eye-safe" wavelength region.
For example, by using methane with a frequency shift of 2916 cm.sup.31, a Nd:YAG laser operating at a wavelength of 1.06 .mu.m can have its output converted to the "eye-safe" 1.54 .mu.m wavelength. The 1.06 .mu.m laser radiation is coupled into a resonator containing the methane Raman medium and produces scattered radiation at the 1.54 .mu.m wavelength.
Raman shifted lasers such as those described in U.S. Pat. Nos. 4,103,179, issued to W. Schniedt, and 3,668,420 issued to J. T. Vanderslice use a Raman cell resonator to convert 1.06 .mu.m wavelength radiation from a pump laser to 1.54 .mu.m . The Raman resonator of each of these patents has a pressurized gas medium disposed along an optical path between two mirrors. One mirror, the input mirror, is substantially totally transmissive at 1.06 .mu.m and substantially totally reflective at 1.54 .mu.m . This allows 1.06 .mu.m radiation to enter the cell but does not allow 1.54 .mu.m radiation back into the pumping laser. The second mirror, the output mirror, is partially reflective at 1.54 .mu.m and substantially totally reflective at 1.06 .mu.m. This configuration allows the output of 1.54 .mu.m radiation from the Raman resonator but traps the 1.06 .mu.m pumping radiation. However, this Raman resonator technique is not without its limitations.
The Raman scattering process is intensity dependent. Therefore, any decrease in pump radiation intensity lessens the conversion efficiency of radiation to the new wavelength. Improper alignment, spacing or curvature of the mirrors in the Raman cell resonator causes the radiation to diverge from central axis or degrade the focus within the Raman cell. This in turn lowers the intensity and thus decreases conversion. The Raman resonator formed by mirrors also needs to be precisely aligned with the pump laser and associated optics to insure optimum radiation transfer into the Raman resonator and maintain maximum pump intensity along the focused optical path.
In addition, other scattering processes such as Stimulated Brilluoin Scattering (SBS) can greatly decrease the wavelength conversion efficiency. The SBS radiation returns through the Raman medium and the input mirror to the pump laser. Large enough amounts of radiation returning to the pump laser causes operational problems or damage.
SBS normally occurs to some extent within the Raman medium but is greatly enhanced for misaligned optics. The SBS and Raman scattering processes are in direct competition within the medium. The threshold for onset of stimulated Raman scattering (SRS) must be below the SBS threshold so that SRS occurs first, and energy is depleted from the medium by transfer into radiation at the desired Raman wavelength. However, misaligned optics increase the SRS threshold by detuning the Raman resonator and by causing non-overlapping paths for the incident pump radiation and Raman scattered radiation.
The SBS is reflected back into the pump laser and can damage the pump laser or severely impact on its performances.
What is needed is a method and apparatus to ensure good alignment of the driver laser and Raman cell and simplification of the overall Raman laser.